Distillate fuels derived from Fischer-Tropsch processes are often hydrotreated to eliminate unsaturated materials, e.g., olefins, and most, if not all, oxygenates. The hydrotreating step is often combined with mild hydroisomerization resulting in the formation of iso-paraffins, often necessary for meeting pour point specifications for distillate fuels, particularly fuels heavier than gasoline, e.g., diesel and jet fuels.
Fischer-Tropsch distillates, by their nature, have essentially nil sulfur and nitrogen, these elements having been removed upstream of the Fischer-Tropsch reaction because they are poisons, even in rather small amounts, for known Fischer-Tropsch catalysts. As a consequence, Fischer-Tropsch derived distillate fuels are inherently stable, the compounds leading to instability, e.g., by oxidation, having been removed either upstream of the reaction or downstream in subsequent hydrotreating steps. While stable, these distillates have no inherent inhibitors for maintaining oxidative stability. Thus, upon the onset of oxidation, as in the formation of peroxides, a measure of oxidative stability, the distillate has no inherent mechanism for inhibiting oxidation. These materials may be viewed as having a relatively long induction period for oxidation, but upon initiation of oxidation, the material efficiently propagates oxidation.
The development of gas fields, i.e., where the gas is natural gas and primarily contains methane, often includes the recovery of gas field condensates, hydrocarbon containing liquids associated with the gas. The condensate normally contains sulfur but not in a form that usually acts as an inhibitor. Gas field condensates thus have relatively short induction periods but are inefficient for propagating oxidation. Thus, the condensates are often free of thiols or mercaptans which are sulfur containing anti-oxidants.